This section provides background information related to the present disclosure which is not necessarily prior art.
To reduce the quantity of undesirable particulate matter and NOx emitted to the atmosphere during internal combustion engine operation, a number of exhaust aftertreatment systems have been developed. The need for exhaust aftertreatment systems particularly arises when diesel combustion processes are implemented.
One method used to reduce NOx emissions from internal combustion engines is known as selective catalytic reduction (SCR). SCR may include injecting a reagent into the exhaust stream of the engine to form a reagent and exhaust gas mixture that is subsequently passed through a reactor containing a catalyst, such as, activated carbon, or metals, such as platinum, vanadium, or tungsten, which are capable of reducing the NOx concentration in the presence of the reagent.
An aqueous urea solution is known to be an effective reagent in SCR systems for diesel engines. However, use of an aqueous solution and other reagents may include disadvantages. Urea is highly corrosive and attacks mechanical components of the SCR system. Urea also tends to solidify upon prolonged exposure to high temperatures, such as encountered in diesel exhaust systems. A concern exists because the reagent that creates a deposit is not used to reduce the NOx.
In addition, if the reagent is not properly mixed with the exhaust gas, the reagent is not efficiently utilized, inhibiting the action of the catalyst and thereby reducing the SCR system's effectiveness. High reagent injection pressures have been used as a method of minimizing the problem of insufficient atomization of the urea mixture. However, high injection pressures may result in over-penetration of the injected spray plume into the exhaust stream thereby causing the plume to impinge on the inner surface of the exhaust pipe opposite the injector. Over-penetration leads to inefficient use of the urea mixture and may reduce the range over which the vehicle may be operated with reduced NO emissions. Only a finite amount of reagent may be carried in a vehicle. It is desirable to efficiently use the stored reagent to maximize vehicle range and reduce the need for replenishing the reagent.
It may be advantageous to provide methods and apparatus for injecting a reagent into the exhaust stream of an internal combustion engine to minimize reagent deposition and improve the mixing of the reagent with the exhaust gas.